The present invention relates to a tire for vehicle wheels comprising: a carcass structure having at least one carcass ply comprising thread elements each extending in a substantially "U"-shaped conformation about the profile in transverse section of the tire, and a pair of circumferentially inextensible annular structures each engaged close to a respective inner circumferential edge of the carcass ply; optionally, a belt structure applied to the carcass structure at a circumferentially outer position thereof; a tread band applied to the belt structure at a circumferentially outer position thereof; at least one pair of sidewalls applied to the carcass structure at laterally opposite positions.
Manufacture of tires for vehicle wheels involves formation of a carcass structure essentially consisting of one or more carcass plies substantially having a toroidal conformation and the axially opposite side edges of which engage respective circumferentially inextensible annular reinforcing elements called "bead cores".
Applied to the carcass structure, at a circumferentially external position thereof, is a belt structure comprising one or more belt strips in the form of a closed ring, essentially consisting of textile or metal cords suitably oriented relative to each other and to the cords belonging to the adjacent carcass plies.
A tread band currently consisting of a strip of elastomer material of appropriate thickness is applied to the belt structure, at a circumferentially external position thereof.
It is to point out that, to the aims of the present invention, by the term "elastomer material" it is intended a rubber blend in its entirety, that is the assembly made up of a base polymer suitably amalgamated with mineral fillers and/or additives of any other type.
Finally, to the opposite sides of the tire being manufactured a pair of sidewalls are applied, each of them covering a side portion of the tire included between a so-called shoulder region, located close to the corresponding side edge of the tread band, and a so-called bead located at the corresponding bead core.
In accordance with traditional production methods, essentially the above listed tire components are first made separately from each other to be then assembled during a tire-manufacturing step.
For instance, for making the carcass ply or plies to be associated with the bead cores to form the carcass structure, production of a rubberized fabric comprising longitudinally-disposed continuous textile or metallic cords, through an extrusion and/or calendering process, is first required. This rubberized fabric is submitted to a transverse-cutting operation to produce lengths of predetermined sizes that are subsequently joined together so as to give rise to a continuous semifinished product having transversely-disposed parallel cords.
Then this article of manufacture is to be cut into pieces the length of which is correlated with the circumferential extension of the carcass to be made.
Production methods have been recently proposed which, instead of resorting to the production of semifinished products, make the carcass structure directly during the tire-manufacturing step.
For example, U.S. Pat. No. 5,453,140 herein quoted as an example of the most pertinent state of the art, discloses a method and an apparatus forming a carcass ply starting from a single cord previously wound around a reel.
According to the method and apparatus described in the above patent, at each operating cycle of the apparatus the cord drawn from the reel by power-driven idler rollers and maintained taut by a pneumatic tensioning system is cut to size to obtain a length of preestablished extension.
The cord length is picked up by a pick-up element mounted on a belt wound over power-driven pulleys to be extended transversely on the outer surface of a toroidal support.
The length ends are then engaged by bending members of the belt type operating on the opposite sides of the toroidal support for radially applying the cord length to the toroidal support by means of slide elements acting like fingers along the side portions of the cord length.
Repetition of the above described operating cycle leads to deposition of a number of cord lengths disposed circumferentially in side-by-side relationship until the whole circumferential extension of the toroidal support has been covered.
Necessarily, the toroidal support is previously coated with a layer of raw rubber having a dual function, i.e. that of adhering to the cords deposited thereon so as to conveniently hold them in a fixed positioning, and that of constituting an inner liner impervious to air in the finished tire.
Tires obtained by this production method have a carcass structure in which the carcass ply or plies consist of individual cords each having two side portions axially spaced apart from each other and oriented radially of the rotation axis of the tire, and a crown portion extending at a radially outer position between the side portions.
Within the scope of the carcass structure manufacture, it is also known that close to each of the tire beads, the opposite ends of the individual cords forming a carcass ply are located, in an alternating sequence, at axially opposite positions relative to an annular anchoring element forming said bead core, as can be learned from patent EP 0 664 231.
In accordance with the present invention, within the tire-manufacturing scope remarkable advantages have been found to be achieved if the carcass ply or plies are made by depositing strip lengths instead of individual cords, each of said lengths essentially comprising a layer of raw elastomer material incorporating two or more parallel thread elements disposed longitudinally.